1. Field of the Invention:
This invention relates to a travelling wave-type optical modulator.
2. Related Art Statement:
The inventors disclosed in Japanese Laid-open publication Kokai Hei 10-133159(JPA 10-133159) that if the part under an optical waveguide of a substrate to construct a travelling wave-type optical modulator can be reduced up to e.g. 10 xcexcm or below, the optical modulator can be operated at a frequency of 10 GHz or more without a buffer layer made of silicon oxide or the like. In this case, thicker parts and thinner parts are formed in the substrate, and as a result, the travelling wave-type optical modulator can be operated at a high speed and repressed in DC drift. Moreover, the product (Vxcfx80xc2x7L) of operating voltage Vxcfx80 by electrode length L in the optical modulator can be favorably decreased.
Then, the inventors made an attempt to develop the performances of the above optical modulator, and discovered the following problem. That is, in the case of connecting the above optical modulator to an external optical fiber, the optical insertion loss was increased. Moreover, it is desired that the product (Vxcfx80xc2x7L) is more decreased with keeping the velocity matching for microwave signal and the impedance matching for an external circuit.
It is an object of the present invention, in a travelling wave-type optical modulator including a substrate made of a ferroelectric single crystal having thicker and thinner parts, to decrease the optical insertion loss with keeping the velocity matching for microwave signal and the impedance matching for an external circuit.
It is another object of the present invention, in a travelling wave-type optical modulator including a substrate made of a ferroelectric single crystal having thicker and thinner parts, to greatly decrease the product (Vxcfx80xc2x7L) of operating voltage Vxcfx80 by electrode length L with keeping the velocity matching for microwave signal and the impedance matching for an external circuit.
The present invention is a travelling wave-type optical modulator including a supporting substrate having a ferroelectric single crystalline layer positioned thereon. The ferroelectric single crystalline layer includes thicker and thinner parts within the modulating region of the travelling wave-type optical modulator when viewed in the cross section of the modulating region. An optical waveguide is formed in the thicker part of the ferroelectric single crystalline layer. Electrodes for modulation are provided on the thinner parts of the ferroelectric single crystalline layer between the adjacent thicker parts.
A method for manufacturing the travelling wave-type optical modulator of the present invention, includes the following steps. A substrate made of a ferroelectric single crystalline material is prepared and an optical waveguide is formed in the substrate. The substrate is adhered to another supporting substrate and processed to have thicker and thinner parts, when viewed in the cross section of the modulating region, within the modulating region of the travelling wave-type optical modulator. The optical waveguide is positioned in the thicker part of the substrate. Electrodes are provided for modulation, each being positioned on the thinner parts between the adjacent thicker parts of the ferroelectric single crystalline layer.
According to the above travelling wave-type optical modulator, the optical insertion loss can be decreased while keeping the velocity matching for microwave signal and the impedance matching for an external circuit as described below.
The optical waveguide can be formed in any area within the thicker part of the ferroelectric single crystalline layer. Preferably, the optical waveguide is formed in an upper area of the thicker part or a lower part thereof adjacent to the supporting substrate. If the waveguide is formed in the lower area, the product (Vxcfx80xc2x7L) can be decreased more effectively.
In a preferred embodiment of the present invention, the supporting substrate includes a base substance made of a hard material and an adhesive layer on the base substance. As the hard material, a ferroelectric single crystal as described below, a glass material and a resin material are exemplified. The adhesive layer may be made of a glass material having lower dielectric constant and an operation temperature of about 600xc2x0 C. or below. Concretely, solder glass made of plural glass materials such as silicon oxide, lead oxide, aluminum oxide, magnesium oxide, calcium oxide, and boron oxide is exemplified.
As the resin material, room temperature-cured resin, heat-cured resin or ultraviolet-cured resin are exemplified. The resin materials also preferably have lower dielectric constants.
Another aspect of this invention relates to a travelling wave-type optical modulator including a supporting substrate having ferroelectric single crystalline portions positioned on the supporting substrate, each being separated within the modulating region of the travelling wave-type optical modulator when viewed in the cross section of the modulating region. An optical waveguide is formed in the ferroelectric single crystalline portion, and electrodes are provided for modulation, each being provided between the adjacent ferroelectric single crystalline portions on the supporting substrate.
A method for manufacturing the above travelling wave-type optical modulator includes the following steps. A substrate made of a ferroelectric single crystalline material is prepared and an optical waveguide is formed in the substrate. The substrate is adhered to another supporting substrate and processed to fabricate ferroelectric single crystalline portions, each being separated, when viewed in the cross section of the modulating region, within the modulating region of the travelling wave-type optical modulator. As such, the optical waveguide is positioned in the ferroelectric single crystalline portion. Electrodes are provided for modulation, each being positioned between the adjacent ferroelectric single crystalline portions.
According to the above travelling wave-type optical modulator, the product (Vxcfx80xc2x7L) of operating voltage Vxcfx80 by electrode length L can be much decreased while keeping the velocity matching for microwave signal and the impedance matching for an external circuit, as described below.
Although the configuration of each ferroelectric single crystalline part is not restricted, it is required that the adjacent ferroelectric single crystalline portions are separated from one another, and the electrodes are formed in the spaces between the adjacent ferroelectric single crystalline portions. Moreover, it is desired that the ferroelectric single crystalline portion is constructed to have a bottom surface adjacent to the supporting substrate, a top surface opposite to the supporting substrate and side surfaces positioned between the bottom surface and the top surface. In this case, each electrode is arranged so as to be contacted with the opposing side surfaces of the adjacent ferroelectric single crystalline parts. Moreover, it is preferable that the top surface is parallel to the bottom surface.
Particularly, in this case, it is desired that each electrode is contacted with the top surfaces of the adjacent ferroelectric single crystalline portions.
The optical waveguide may be formed in any area within the ferroelectric single crystalline portion. For example, the optical waveguide may be formed in the upper area or the lower area of the ferroelectric single crystalline portion. If the optical waveguide is formed in the lower area, the product (Vxcfx80xc2x7L) of operating voltage Vxcfx80 by electrode length L can be greatly decreased.
In the above travelling wave-type optical modulator, the supporting substrate preferably includes a base substance made of a hard material and an adhesive layer on the base substance. As the hard material, a ferroelectric single crystal, a glass material and a resin material are exemplified, as mentioned above. The adhesive layer may be also made of the above-mentioned solder glass. Moreover, the supporting substrate may be made of a glass material or a resin material entirely.
The ferroelectric single crystalline portion is made of any material used in the known travelling wave-type optical modulator, and concretely, lithium niobate, potassium lithium niobate, lithium tantalate and KTP are exemplified. Preferably, at least one of lithium niobate, lithium tantalate and lithium niobate-lithium tantalate solid solution single crystal is employed.